A Time Division Multiplexing (TDM) digital switch typically contains three main components. A data memory receives and stores data received from serial input streams. A connection memory stores the desired connection paths and connection characteristics. A hardware switching unit sequentially reads the connection memory, and then transfers the data from the data memory to output streams according to the desired connection characteristics. This is fundamentally how the digital switching components of the switches described in U.S. Pat. No. 4,510,597, issued to Lewis on Dec. 3, 1981, and in U.S. Pat. No. 6,507,579 issued to Gresham on Jun. 25, 1999, work.
The digital switches taught by '597 and '579 switch input streams having a constant data rate. With the evolution of digital switches and the creation of standards for formatting of serial streams, TDM digital switches commonly incorporate features that provide a programmable serial data rate, thereby allowing a more versatile interface. For example, the MT90869™ digital switch produced by Zarlink Semiconductor Inc., allows programmable data rates.
However, simply varying the data rate of input streams can lead to underutilization of connection memory and data memory. The data memory and the connection memory of a device must be large enough to carry the data with streams operating at a maximum rate. If the data rates of the streams are reduced, portions of the data memory and of the connection memory will remain unused. The digital switch will therefore be operating below maximum capacity, and the traffic through the device will be reduced. For example, if the data rate of each stream is reduced to half the maximum data rate, only half the data memory will be in use at any time.
This loss of bandwidth and redundancy of memory at lower data rates is normally considered part of the cost of the feature of providing programmable input and output data rates. A digital switch that allowed programmable data rates in the serial input and output streams while using as much connection memory and data memory as possible would result in improved bandwidth utilization.